This invention relates to capacitive liquid level sensors used to control the sampling of liquids from a container. It also relates to capacitive sensors capable of sensing the interface between different layers, including air. Such liquid level sensors find use in many instruments wherein a robotic probe is used to withdraw liquid from a container, containing a sample to be analyzed or a reagent, in a controlled manner.
In such robotic systems, it is desirable to have knowledge of the level of the liquid at the interface between liquids in the container such that the probe used to withdraw the liquid can be controlled (1) to withdraw a particular layer of liquid in the container or (2) to minimize contact with an undesired portion or layer of the liquids in the container. In such systems, one is dealing with generally separable fluids such as occurs in the collection of blood. In a typical blood system producing packed red blood cells, the red blood cells will be in the lower portion of the container. Immediately above the packed cells is a commercial separation gel as described in U.S. Pat. No. 3,852,194 to Zine. Above the separation gel is the serum or plasma and finally air is on the top. Contamination of the sampling probe by the separation gel is very undesirable. Often it is relatively difficult to remove the gel from the probe and the gel can in fact cause clogging of the probe and missampling to occur. Therefore it is highly desirable to provide some system of locating the gel-serum interface so that the serum only can be withdrawn and the probe prevented from contacting the gel.
To accomplish this objective, it is necessary to be able to sense the level of the liquid interfaces on a real time basis. Various level sensors have been developed for this purpose. Among those are the so-called capacitive level sensors. These are based on the fact that any conductor (say a sensing probe) exhibits a finite electrical capacitance. This capacitance, when approaching a liquid having a higher dielectric constant, will increase. When the sensing probe is in close proximity to a liquid, the higher dielectric constant and greater surface area results in an increased capacitance of the probe. These capacitance changes caused by the liquid can be rather small so that sensitive detection devices are required.
Devices known in the prior art that are suitable for detecting small changes in capacitance include bridges, RC or LC oscillators and frequency meter counters (including heterodyning), phase locked loops, zero crossing periodometers, amplitude changes to an RC or LC filter, and phase shift changes through an RC or LC circuit.
Among the prior art capacitive liquid level sensors is Kingston U.S. Pat. No. 3,391,547 which discloses a capacitive liquid level probe for a liquid tank. He utilizes a capacitor probe, disposed in the liquid, as one leg of a bridge circuit. An unbalance in the circuit, as a result of change in capacitance of the probe, is detected by a phase sensitive detector which is referenced by the fixed frequency excitation oscillator through a variable phase shifter. The variable phase shifter allows for offset adjustment.
In similar manner, Oberli U.S. Pat. No. 3,635,094, discloses a capacitive level sense means for an automatic transfer pipette. The sample probe is utilized as the first element and a metal stand around the sample vessel is the second element which forms a capacitor in one leg of a bridge circuit. The remaining legs of the bridge consist of a variable capacitor leg and two resistor legs. The variable capacitor leg may be adjusted such that its capacitance matches that of the probe contacting the liquid. The bridge circuit is excited by a fixed frequency oscillator and a differential amplifier is utilized to determine when the bridge is balanced indicating that the probe has contacted the liquid.
Finally, Shimizu U.S. Pat. No. 4,818,492 discloses a capacitive liquid level sensor for an automatic clinical analyzer. The output is filtered and compared against a reference value to provide a signal indicating the presence of liquid at the probe.
None of these sensors are directed to sensing the liquid interfaces in any useful fashion as the probe must disturb such interfaces as it journeys down through the tube or container. To solve this problem, various systems have been devices which seek to determine liquid level from the exterior of a container. Typical of these systems are those described in U.S. Pat. No. 4,099,167 and U.S. Pat. No. 4,002,996. In both of these systems electrodes are disposed on the exterior of the container and changes in the dielectric provided by the contained liquid as compared to air is sensed by causing a variation in a capacitance sensitive detector. Another system such that described in U.S. Pat. No. 4,371,790 uses the electrical conductance of a liquid to determine the level of the liquid contained in a container.
Finally, U.S. Pat. No. 3,939,360 describes a similar system in which a tape is attached or fixed to the outside of a container whose liquid level is to be sensed. Unfortunately such system is unable to seek the level of liquid/air interface but must allow the interface to pass by its location before such is detected.
The problem with these latter interface sensors is that while they are able to sense the interface between liquid levels, they are not able to ascertain whether there is sufficient fluid in a particular layer to sample, whether there was some error in creating the liquid levels which resulted in the absence of a particular layer, or whether the liquid container is normal. These additional factors must be ascertained if adequate sampling is to be accomplished.